1. Technical Field
This invention pertains generally to magneto-resistive (MR) based sensor assemblies, and more particularly, to a control circuit for use therewith configured to generate an output signal indicative of an angular position of a rotating member.
2. Description of the Related Art
Digital magnetic position sensors are devices that are significant to many industries, including the automotive industry. Such devices are used to sense an angular position of a shaft, such as a crankshaft or a camshaft of an engine. Information on the shaft position may then be used for fuel and ignition timing, and the like. In one application a very high degree of angular accuracy and repeatability is required to detect small variations in crankshaft rotations, for example, less than 0.050 degrees, for misfire detection. Methods are known for making such determinations, such as set forth in U.S. Pat. No. 5,754,042 entitled xe2x80x9cMAGNETORESISTIVE ENCODER FOR TRACKING THE ANGULAR POSITION OF A ROTATING FERROMAGNETIC TARGET WHEELxe2x80x9d issued to Schroeder et al.
Schroeder et al. disclose an apparatus for detecting angular positions of a rotating object (e.g., shaft). The apparatus includes a magnet, and two magneto-resistive (MR) sensors positioned between the magnet and a target wheel that is attached to the rotating object. The target wheel (both single-track and dual-track embodiments are disclosed) has a plurality of teeth separated by slots angularly spaced around the periphery. Constant current sources electrically bias the two MR sensors. When the target wheel rotates, the resistance of each MR sensor changes due to varying magnetic fields to which the MRs are exposed. The varying resistance is operative to generate an analog voltage signal that transitions between two voltage levels at the passage of the leading and trailing edges of the teeth. The two sensor signals are processed to output a digital signal having state transitions corresponding to the tooth edges. Implementing the system disclosed in Schroeder et al., however, presents certain challenges.
The accuracy of the detector of Schroeder et al. depends, to some degree, on using accurately matched MR sensors. The MR sensors have an inherent tendency for a mismatch in their resistance characteristics due to a number of factors. The mismatch leads to analog signals that vary in such a way that the accuracy (i.e., the degree to which digital output edges correspond to tooth/slot features) is affected. One factor involves variations in the manufacturing process of the MR sensors. Another factor involves subtle differences in a magnetic bias field to which the MRs are exposed. The differences in the magnetic bias field result from a variety of factors, including fluctuations in an air-gap (i.e., a distance between the MR sensor and the peripheral surface of a target wheel tooth), and imperfections in the target wheel, and bias magnet themselves. Yet another factor involves temperature. In particular, the resistance profile of an MR sensor can vary greatly over a temperature range encountered in the automotive environment, For example, xe2x88x9240xc2x0 C. to +180xc2x0 C. Worse yet, the variation in the resistance profile differs from MR sensor to MR sensor. Other factors leading to mismatch include stress and aging. In all, the analog voltage signal produced by the MR sensors have variations in its amplitude, shape and DC offset voltage that are induced by external factors that cannot be controlled or are difficult and/or costly to control.
One general approach taken in the art to deal with signal variability due to mismatch purports to improve the matching of the MR sensors. For example, one particular approach taken in the art involves pretesting multiple MR sensors to enable selection of sensors that are closely matched. However, this approach increases cost, is relatively time consuming and in any event is difficult to achieve desired levels of match over the wide temperature range encountered. A related approach is disclosed in U.S. Pat. No. 5,916,459, issued to Schroeder et al., entitled xe2x80x9cMETHOD OF MATCHING MAGNETORESISTORS IN A SENSOR ASSEMBLY,xe2x80x9d which involves use of a laser to reduce the magnetic field portion of a permanent magnet underlying the sensor with a higher electrical resistance. This particular approach may also increases cost and time.
Another general approach taken in the art to deal with signal variability due to mismatch does not involve trying to improve the sensor match but rather involves the downstream signal processing. For example, one particular approach involves the use of an adaptive threshold, as seen by reference to U.S. Pat. No. 5,917,320 issued to Scheller et al. entitled xe2x80x9cDETECTION OF PASSING MAGNETIC ARTICLES WHILE PERIODICALLY ADAPTING DETECTION THRESHOLDxe2x80x9d. Scheller et al. detects a peak-to-peak level of an input sensor signal, and then produces a threshold signal that is a percentage of the peak-to-peak level. A digital output signal is generated by comparing the threshold signal and the input sensor signal. However, a broad applicability of Scheller et al. is limited inasmuch as the processing circuits assume a relatively large and stable input signal. In particular, Scheller et al., does not disclose the use of one/two MR sensors, but rather discloses in one embodiment the use of a MR sensor bridge as a magnetic field-to-voltage converter. It is known that a MR sensor bridge minimizes many variabilities in the output, especially as to temperature. However, such a bridge requires four (4) MR sensors, which is prohibitively expensive in many applications.
Moreover, simple AC coupling (e.g., series connected capacitor to filter DC) has two problems. First, it introduces a phase error, which is unacceptable when trying to align edges of the digital output signal with the corresponding edges of the teeth/slot features. Second, it does not perform at low frequencies (e.g.,  less than 20 HZ), as contemplated in automotive applications.
There is therefore a need for a control circuit for an MR sensor based assembly that minimizes or eliminates one or more shortcomings as set forth above.
One advantage of the present invention is that it provides, in one embodiment, accurate angular position indications using only one magneto-resistive (MR) magnetic field sensor, thereby providing a reduced-cost product. As a result, the relatively costly and time consuming MR sensor matching approaches need not be employed. For improved temperature compensation, a two-sensor embodiment, which also provides accurate detection, is provided.
In accordance with the present invention, an apparatus is provided for generating an output signal indicative of an angular position of a rotating member. The apparatus includes a target wheel having a plurality of teeth separated by slots angularly spaced around a periphery thereof. The target wheel is configured to be mounted to the rotating object for rotation therewith. The apparatus further includes a sense assembly having a magnetic field biasing device such as a magnet, and a magneto-resistive (MR) magnetic field sensor disposed between the magnet and the target wheel. The apparatus also includes a control circuit coupled to the MR sensor and configured to generate the output signal, which has transitions between first and second states at the passage of each leading and trailing edge of the teeth of the target wheel. The control circuit also includes an arrangement for electrically biasing the MR sensor so as to produce an input voltage signal for further processing. The input signal has a direct-current (DC) component and an alternating-current (AC) component superimposed thereon when the target wheel rotates. In accordance with the present invention, the control circuit further includes a first amplifier circuit configured to substantially remove the DC component from the input signal and to amplify the input signal by a predetermined factor selected to maximize the AC component within a dynamic range of the control circuit. Through the foregoing, a relatively small AC component input is positioned and amplified within the dynamic range of the control circuit, which simplifies and optimizes generation of a threshold signal used in generating the output signal.
In a preferred embodiment, the control circuit further includes a peak detector circuit responsive to the output of the first amplifier circuit (i.e., a first signal) for generating a second signal representative of a maximum voltage level of the first signal. A second amplifier circuit, responsive to the first signal and the second signal, is configured to generate a third signal having any remaining DC component substantially removed. A reference threshold generator produces a reference threshold signal corresponding to a predetermined percentage of a peak level of the third signal. A Comparator is provided to generate the final output signal by comparing the reference threshold signal and the third signal. The reference threshold signal establishes the transition level for the output signal that will correspond to the true position of the tooth edges on the target wheel. Accurate and repeatable detection of the edges is desirable for many applications, including crankshaft and camshaft position determination.